1. Field of the Invention
The present invention relates a solid electrolytic capacitor and a method of making the same and, in particular to the solid electrolytic capacitor of a kind having a solid electrolytic layer made of an electroconductive polymer, and the method of making the same.
2. Description of the Prior Art
With the advent of electronic appliances that are high-frequency oriented, large capacitance electrolytic capacitors that are an electronic component part are desired to have an excellent impedance characteristic (hereinafter referred to as ESR characteristic) in a high frequency region.
A solid electrolytic capacitor is not an exception and, in order to realize this, a surface condition of an anode, a method of forming a dielectric oxide film, improvement of electrolyte, a surface condition of a cathode and the structure of a capacitor element have been studied and examined.
FIG. 12 illustrates a sectional representation of a standard solid electrolytic capacitor 50. The solid electrolytic capacitor 50 includes a capacitor element 25 embedded in a package 29 generally made of an epoxy resin and reinforcing resin 30 with respective portions of anode and cathode terminals 26 and 27 exposed to the outside.
The capacitor element 25 is made up of a porous anode element 20, a dielectric oxide film 22 formed on a surface of the anode element 20, a solid electrolytic layer 23 formed over the dielectric oxide film 22 and a cathode layer 24 formed over the solid electrolytic layer 23.
The porous anode element 20 is obtained by press-shaping a metal powder of tantalum which is a valve action metal to a desired shape and then sintering it, and the anode element 20 has embedded therein an anode lead line 21 made of a tantalum wire. The anode lead line 21 is connected with the anode terminal 26.
On the other hand, the cathode layer 24 is formed by laminating a carbon and a silver paint and is connected with the cathode terminal 27 through an electroconductive bonding agent 28.
A method of making the standard solid electrolytic capacitor 50 will be discussed with reference to FIG. 13 in which there is shown a flowchart showing the sequence of making the solid electrolytic capacitor 50 according to the prior art. As shown therein the tantalum metal powder with the anode lead line 21 in the form of the tantalum wire embedded therein is press-shaped to a desired shape and is then sintered to provide the porous anode element 20.
Subsequently, using phosphoric acid, the anode element 20 is anodized to form the dielectric oxide film 22 on an outer surface of the anode element 20 (Anodizing Step).
Thereafter, after the anode element 20 has been impregnated with a pyrrole monomer solution, the anode element 20 is dipped into an oxidizer solution containing iron(l) p-toluenesulfonic acid, iron dodecylbenzenesulfonic acid and so on so that the solid electrolytic layer 23 can be formed over the dielectric oxide film 22 by means of a chemical oxidation polymerization. See, the Japanese Laid-open Patent Publications No. 60-244017 and No. 63-181308.
The porous anode element 20 is repeatedly dipped into the monomer solution and then into the oxidizer solution as disclosed in, for example, U.S. Pat. No. 4,697,001 to form the solid electrolytic layer 23 on the outer surface of the anode element 20 and also within micropores of the anode element 20.
For the solid electrolytic layer 23, other than pyrrole, an electroconductive polymer formed by polymerization of thiophene, which is a heterocyclic compound, or furan is employed. Since the above described electroconductive polymer has a very low solid resistance, development has been made with the electroconductive polymer regarded as an effective compound to reduce the impedance of the solid electrolytic capacitor and is put into practical use.
Thus, by means of the chemical oxidation polymerization, the solid electrolytic layer 23 made of polypyrrole is formed on the dielectric oxide film 22 on the anode element 20 (Polymerizing Step).
Thereafter, carbon is coated, a silver paint is coated and drying is performed to complete formation of the cathode layer 24, thereby completing the capacitor element 25 (Cathode Layer Forming Step).
Then, the anode lead line 21 extending from the capacitor element 25 is soldered to the anode terminal 26 of a COM terminal and the cathode layer 24 is connected with the cathode terminal 27 through an electroconductive bonding agent 28 (Fabricating Step). The capacitor element 25 is thereafter resin-molded in an epoxy covering resin 29 with respective portions of the anode and cathode terminals 26 and 27 exposed to the outside of the covering resin 29 (Resin-encasing Step). In general, by the sequence discussed above, a batch of capacitors 50 are manufactured at a time with the anode and cathode terminals 26 and 27 of one capacitor 50 continued to those of the next adjacent capacitor 50. Accordingly, as a final step, the capacitors 50 connected together are separated into the individual capacitors 50 which are subsequently tested to provide the individual solid electrolytic capacitors 50 (Finishing Step).
However, the prior art capacitor making method discussed above has the following problems which occur during the polymerizing step in which the solid electrolytic layer 23 is formed.
In the first place, since the chemical oxidation polymerization is repeated a number of times to form the solid electrolytic layer 23 on the outer surface of the anode element 20 and within the micropores of the anode element 20, residues 31 of solid electrolyte tend to be formed on the outer surface of the anode element 20 as shown in FIG. 14 and within the micropores 20P of the anode element 20 as shown in FIG. 15.
It is to be noted that FIGS. 14 and 15 illustrate the anode element 20 obtained after the polymerization step discussed above. Although not shown in FIG. 14, the surface of the anode element 20 is formed with the oxide film 22 and the solid electrolytic layer 23. The plural anode elements 20 are connected to a support bar 3 by means of the respective anode lead lines 21 connected therewith. FIG. 14 makes it clear that the residues 31 are formed on the outer surface of the anode element 20 having the solid electrolytic layer 23. Also, FIG. 15 is a fragmentary enlarged diagram of a portion of the anode element 20, and it makes clear that the residues 31 are formed within the micropores 20P of the anode element 20 formed with the solid electrolytic layer 23. It is to be noted that although not shown in FIG. 15, the dielectric oxide film 22 is formed on the surface of the anode element 20.
The residues 31 of the electrolyte referred to above are made up of lees left during the chemical oxidation polymerization, unpolymerized electroconductive polymer and/or oxidizing agent and they do not only deteriorate an outer appearance of the capacitor element 25 to reduce the volumetric capacity and, hence, to reduce the capacitor characteristic, but may often leak out of the covering resin 29 in the worst case it may occur. It is noted that the term xe2x80x9cvolumetric capacityxe2x80x9d means the degree of ease of encasing the capacitor element within, for example, an epoxy covering resin. Hence, when it comes to a high volumetric capacity, it means that the capacitor element is completely encased easily. Accordingly, in order to remove the residues 31, the use has been made of a brush or the like to remove the residues 31 prior to the cathode layer forming step to render the surface of the solid electrolytic layer 23 to be flat and to repair the outer shape, resulting in reduction in productivity. Also, depending on the condition under which the residues 31 are removed, the solid electrolytic layer 23 may be damaged, resulting in deterioration of the capacitor characteristic.
Secondly, since during the step of forming the solid electrolytic layer 23, the anode element 20 dipped into one of the monomer solution and the oxidizer solution has to be subsequently dipped into the other of the monomer solution and the oxidizer solution, when the anode element 20 dipped into one of the monomer solution and the oxidizer solution is to be dipped into the other of the monomer solution and the oxidizer solution, the solution impregnated in the anode element 20 tends to be diffused into the other solution. Once this occurs, the concentration of each of those solutions particularly within the micropores 20P of the anode element 20 may decrease, resulting in reduction of the coated amount of the solid electrolytic layer 23 and, therefore, deterioration occurs in the capacitance characteristic and the impedance characteristic of the capacitor.
Accordingly, in order form the surface of the porous anode element 20 having its surface formed with the dielectric oxide film 23 to be covered by the continuous solid electrolytic layer 23 so that the intrinsic capacitance can be completely delivered and also to provide the solid electrolytic capacitor capable of exhibiting a low ESR, the step of polymerization necessitated to form the solid electrolytic layer 23 has to be repeated several tens times, resulting in considerable reduction in productivity.
Also, there has been a problem in that the solid electrolytic layer 23 formed by repeating the polymerization step as described above tends to exhibit a high resistance among the solid electrolytic layers 23 and the ESR characteristic does not improve.
Finally, in a method in which the solid electrolytic layer 23 is formed by the chemical oxidation polymerization using the monomer solution and the oxidizer solution, trivalent and bivalent iron ions of the oxidizing agent which did not contribute to the polymerization remain in the solid electrolytic layer 23 after the polymerization. Where the solid electrolytic layer 23 is formed over a detect of the dielectric oxide film 22, those iron ions are reduced to iron under influence of a oxidation reducing potential difference with the dielectric oxide film 22 and will constitute a cause of leakage current and/or shortcircuit, resulting in the yield of production.
In view of the foregoing numerous problems, the present invention has been devised to eliminate the foregoing problems and is to provide a solid electrolytic capacitor and a method of making the same which is effective to exhibit an excellent productivity and capable of providing a highly reliable product.
In order to accomplish the foregoing object, a solid electrolytic capacitor of the present invention is the one including an anode element made of a valve action metal; a dielectric oxide film formed on a surface of the anode element; a solid electrolytic layer formed on a surface of the dielectric oxide film; and a cathode layer formed on a surface of the solid electrolytic layer, which capacitor is featured in that the solid electrolytic layer has an iron concentration not greater than 100 ppm. This type of the solid electrolytic capacitor exhibits a low leakage current and is less susceptible to shortcircuit.
Also, the solid electrolytic capacitor of the present invention may be the one including an anode element made of a valve action metal; a dielectric oxide film formed on a surface of the anode element; a solid electrolytic layer formed on a surface of the dielectric oxide film; a cathode layer formed on a surface of the solid electrolytic layer, which capacitor is featured in that a weight fraction of residues in the solid electrolytic layer is smaller than 5 wt %. This type of the solid electrolytic capacitor is, since the weight fraction of the residues in the solid electrolytic layer is smaller than 5 wt %, excellent in volumetric capacity with deterioration of the ESR suppressed. It is to be noted that the residues referred to above are formed of one or more of substances contained in an unpolymerized monomer solution, substances contained in an oxidizing agent solution, both of which are remaining after the polymerization reaction, and an excessive polymer which is produced more than a desired amount.
Preferably, the solid electrolytic layer is made of polypyrrole and polythiophene.
In accordance with the present invention, there is also provided a method of making a solid electrolytic capacitor which includes the steps of forming a dielectric oxide film on a surface of a porous anode element made of a valve action metal and having a multiplicity of micropores; forming a first solid electrolytic layer over the dielectric oxide film; and forming a cathode layer over the first solid electrolytic layer, which method is featured in that the first solid electrolytic layer forming step includes a substep of forming a first electroconductive polymer film over the dielectric oxide film by contacting the dielectric oxide film with a solution containing a heterocyclic compound and a monomer comprising its derivative to cause the monomer undergo polymerization; a substep of cleansing the first electroconductive polymer film to remove a residue remaining in the first electroconductive polymer film; and a substep of drying the first electroconductive polymer film.
As described above, by removing the residues produced during the polymerization for the first electroconductive polymer film from the first electroconductive polymer film by cleansing the first electroconductive polymer film during the first solid electrolytic layer forming step, the solid electrolytic layer having an uniform thickness can be formed so that the solid electrolytic capacitors of an excellent quality can be manufactured.
Preferably, during the first solid electrolytic layer forming step the first electroconductive polymer film is formed by causing the monomer to undergo a chemical oxidation polymerization using an oxidizing agent. If the first solid electrolytic layer forming step is repeated a number of times, the uniform solid electrolytic layer having a sufficient thickness can be formed.
Also preferably, the substep of cleansing the first electroconductive polymer film includes at least one of a step of removing the residue in the first electroconductive polymer film on a surface of the anode element and a step of removing the residue in the first electroconductive polymer film within the micropores of the anode element. This is because the residue can assuredly be removed from the first electroconductive polymer film.
The step of removing the residue in the first electroconductive polymer film on the surface of the anode element may be carried out by using at least one of (1) a shower cleansing method in which one of a liquid medium including water, hot water and an organic solvent, air and gas, (2) an ultrasonic cleansing method in which the liquid medium is used, (3) a method in which while the anode element is immersed in the liquid medium a voltage is applied with the anode element used as an anode, and (3) blasting.
Preferably, the step of removing the residue in the first electroconductive polymer film within the micropores of the anode element is carried out by using a liquid medium including water, hot water and an organic solvent or an ultrasonic cleansing method using the liquid medium. In such case, the organic solvent may include an organic acid and its salt and has a molecular structure having concurrently a hydroxyl group and a carboxyl group, because by the action of the hydroxyl and carboxyl groups in the molecules the iron ions form a complex that is stable in the solution and, therefore, reduction of the iron concentration from the solid electrolytic layer can be facilitated.
The shower cleansing may carried out by jetting one of the liquid medium, the air and the gas from above or below or the both. In such case, the shower cleansing is preferably carried out by jetting one of the liquid medium, the air and the gas towards the anode element for ten seconds under pressure not lower than 0.5 kg/cm2.
The first solid electrolytic layer forming step may further include a substep of repairing the dielectric oxide film subsequent to the first electroconductive polymer film cleansing substep. In this case, even though the dielectric oxide film is damaged as a result of the cleansing, it can readily be repaired.
Alternatively, the first solid electrolytic layer forming step may further includes a substep of repairing the dielectric oxide film prior to the first electroconductive polymer film cleansing substep. In such case, it is possible to render the dielectric oxide film to be less susceptible to damage which would otherwise be brought about by the cleansing.
Preferably, the first electroconductive polymer film drying substep includes drying the first electroconductive polymer film under vacuum. If the drying is effected under vacuum, the first electroconductive polymer film is free from oxygen deterioration and, therefore, the high-performance, qualitatively stabilized solid electrolytic layer can be formed advantageously.
Subsequent or prior to the first solid electrolyte layer forming step a step of forming a second solid electrolytic layer may be employed and, in such case, the second solid electrolytic layer forming step includes a substep of forming the first electroconductive polymer film using a condition different from a condition used to form the first electroconductive polymer film in the first solid electrolytic layer forming step. This is particularly advantageous in that the solid electrolytic layer can be uniformly formed deep within the micropores of the anode element.
Similarly, subsequent to the first solid electrolyte layer forming step, both a step of forming a second solid electrolytic layer and a step of forming a third solid electrolytic layer may be employed, in which case the second solid electrolytic layer forming step includes a substep of forming the first electroconductive polymer film using a condition different from a condition used to form the first electroconductive polymer film in the first solid electrolytic layer forming step. The third solid electrolytic layer forming step includes a substep of forming a second electroconductive polymer film different from the first electroconductive polymer film.
The first solid electrolytic layer forming step may include a substep of forming the first electroconductive polymer film forming by means of a chemical oxidation polymerization that is effected by dipping the anode element in a solution containing an oxidizing agent (oxidizer solution) and having a pH value not greater than 4 after the anode element has been dipped into the solution containing the monomer. The use of the oxidizer solution of a pH value not greater than 4 is effective to facilitate a reaction speed of the chemical oxidation polymerization and, therefore, when the anode element impregnated with the polymerization solution is subsequently dipped into the oxidizer solution, the chemical oxidation polymerization can be initiated before the polymerization solution dissolves into the oxidizer solution. Therefore, the first solid electrolytic layer having a sufficient thickness can be formed within the micropores of the porous anode element and, consequently, the number of film forming required to form the first solid electrolytic layer can be reduced as compared with that according to the prior art.
The second solid electrolytic layer forming step may include a substep of forming the first electroconductive polymer film forming by means of a chemical oxidation polymerization that is effected by dipping the anode element in a solution containing an oxidizing agent and having a pH value not greater than 4 after the anode element has been dipped into the solution containing the monomer.
It is preferred that the second solid electrolytic layer forming step includes a substep of cleansing the first electroconductive polymer film to remove the residue remaining in the first electroconductive polymer film.
Preferably, the third solid electrolytic layer forming step may include a substep of cleansing the second electroconductive polymer film to remove the residue remaining in the second electroconductive polymer film.
Also preferably, the second solid electrolytic layer forming step includes a substep of dipping the anode element into a solution containing the monomer; a substep of dipping the anode element into a solution containing an oxidizing agent; and a substep of holding in air the anode element which has been removed out of the solution containing the oxidizing agent. In such case, the second solid electrolytic layer forming step is performed at least one time.
The second solid electrolytic layer forming step may also includes a substep of dipping the anode element into a solution containing the monomer; a substep of dipping the anode element into a solution containing an oxidizing agent; and a substep of holding in air the anode element which has been removed out of the solution containing the oxidizing agent, in which case the second solid electrolytic layer forming step is performed at least one time.
Preferably, during the substep of holding in the air the anode element is held in the air of a temperature equal to or higher than a temperature of the solution containing the oxidizing agent.
Furthermore, the third solid electrolytic layer forming step may include a substep of dipping the anode element into a suspension containing the monomer, the oxidizing agent and particles of the second electroconductive polymer; and a substep of holding in air the anode element which has been removed out of the suspension, in which case the third solid electrolytic layer forming step is performed at least one time.
The suspension preferably contains the particles of the second electroconductive polymer prepared by mixing the monomer and the oxidizing agent, and the monomer added after the particles of the second electroconductive polymer have been prepared.
Moreover, a step of heat treating the anode element having the first solid electrolytic layer formed thereon may be carried out prior to or after the cathode layer forming step. This heat treatment is effective to completely remove the organic matter remaining in the solid electrolytic layer and also to immobilize the solid electrolytic layer.
The heat treating step is preferably carried out at a temperature within the range of 200 to 280xc2x0 C.
The first solid electrolytic layer forming step may be carried out intermediate between the second solid electrolytic layer forming step and the third solid electrolytic layer forming step.